1. Field of the Invention
The present invention relates to a device for determining deterioration of a catalyst for an engine. More specifically, the present invention relates to a device which feedback controls the air-fuel ratio of the engine in such a manner that the air-fuel ratio of the exhaust gas flowing into the catalyst changes between lean and rich, compared to the stoichiometric air-fuel ratio, alternately and detects deterioration of the catalyst on the basis of the outputs of air-fuel ratio sensors disposed in the exhaust passage upstream and downstream of the catalyst.
2. Description of the Related Art
A method for determining deterioration of a catalyst on the basis of the output signals of air-fuel ratio sensors disposed in an exhaust gas upstream and downstream of a catalyst of an engine is known. In this method, usually, deterioration of the catalyst is determined on the basis of at least the output signal of the downstream air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing into the catalyst is feedback controlled to a stoichiometric mixture on the basis of at least the output signal of the upstream air-fuel ratio sensor.
A three-way catalyst usually has an O.sub.2 storage capability, i.e., a capability for absorbing oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst is lean compared to the stoichiometric air-fuel ratio and for releasing the absorbed oxygen when the air-fuel ratio of the exhaust gas flowing into the catalyst is rich compared to the stoichiometric air-fuel ratio. Due to this O.sub.2 storage capability, the air-fuel ratio of the exhaust gas flowing out from the catalyst is kept near the stoichiometric air-fuel ratio even when the air-fuel ratio of the exhaust gas flowing into the catalyst fluctuates between a rich air-fuel ratio and a lean air-fuel ratio in a relatively short cycle period. Therefore, if the catalyst does not deteriorate, the fluctuation of the air-fuel ratio of the exhaust gas upstream of the catalyst is smoothed by the catalyst and, thereby, the air-fuel ratio of the exhaust gas downstream of the catalyst fluctuates in a relatively small amplitude and at a relatively low frequency.
However, the O.sub.2 storage capability of the catalyst decreases as the degree of deterioration of the catalyst becomes large, and the amount of oxygen absorbed and released by the catalyst becomes small when the catalyst deteriorates. Therefore, if the catalyst deteriorates, the air-fuel ratio of the exhaust gas downstream of the catalyst fluctuates in the manner similar to that of the air-fuel ratio of the exhaust gas upstream of the catalyst, i.e., when the catalyst deteriorates, the amplitude of the fluctuation of the air-fuel ratio of exhaust gas downstream of the catalyst becomes larger, and the frequency thereof becomes higher. The conventional method utilizes this phenomena for determining the deterioration of the catalyst. Namely, in the conventional method, it is determined that the catalyst deteriorates when the length per unit time of the response curve of the output of the downstream air-fuel ratio sensor becomes longer.
An example of the device which determines deterioration of the catalyst on the basis of the length per unit time of the response curve of the output of the downstream air-fuel ratio sensor is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-98948.
The device in the '948 publication monitors the length per unit time of the response curve of the output of the downstream air-fuel ratio sensor when the air-fuel ratio of the engine is feedback controlled on the basis of the output of the upstream air-fuel ratio sensor in such a manner that the air-fuel ratio of the engine fluctuates regularly around the stoichiometric air-fuel ratio. If the length per unit time of the response curve of the output of the downstream air-fuel sensor becomes longer, the device determines that the catalyst is deteriorated.
If it is determined that the catalyst deteriorates on the basis of the length per unit time of the response curve of the output of the downstream air-fuel ratio sensor, a catalyst which is partially broken may be incorrectly determined as being deteriorated. In this case, the exhaust gas flows through the broken area of the catalyst so that the exhaust gas may not be purified. Therefore, the amplitude of the fluctuation of the output of the downstream air-fuel ratio sensor becomes larger. However, the amplitude of the fluctuation of the output of the downstream air-fuel ratio sensor is not increased to the degree that the catalyst is determined as being deteriorated if the degree of the breakage of the catalyst relative to the entire catalyst is small. Therefore, according to the above device for determining the deterioration of the catalyst on the basis of the length per unit time of the response curve of the output of the downstream air-fuel ratio sensor, a catalyst which is partially broken may not be determined as being deteriorated.
Further, if the downstream air-fuel ratio sensor deteriorates, the amplitude of the output of the downstream air-fuel ratio sensor becomes smaller. Therefore, although the catalyst deteriorates, the amplitude of the fluctuation of the output of the downstream air-fuel ratio sensor is not increased to the degree that the catalyst is determined as being deteriorated. Thus, according to the above device for determining the deterioration of the catalyst on the basis of the length per unit time of the response curve of the output of the downstream air-fuel ratio sensor, a catalyst which is partially broken may not be determined as being deteriorated.